In many communication systems where power is at a premium, such as low-power or battery-powered systems, it is common practice to place the communication nodes in a sleep mode wherein most of the circuits are inactive. In order to establish communications wake-up radios are employed. A wake-up request is sent from a remote instigator to a wake-up receiver of the wake-up radio located at the local node. The wake-up receiver validates the wake-up request by checking that it meets certain predetermined criteria, and in response to a valid wake-up request activates the local receiver so that it can establish communication with the remote transmitter. Typically, two-way transmission is established in which case of course the node will be a transceiver, i.e. including a transmitter.
Wake-up radios are especially useful in communication systems where a low power node is required to start quickly on receipt of an appropriate wake-up signal transmitted from a hub. The initiator of a communication session may be a human or machine user who requires wireless access to the low power node. The expected scheduling of the communication session may be unknown in many applications, and furthermore the actual channel of such a transmission, within a band of channels, may be unknown. Such asynchronous systems that require timely access may use either (i) continuously powered wake-up radios that listen for an incoming signal or (ii) wake-up radios that may listen or “sniff” at intervals for a wake-up signal on each of the potential transmission channels.
Continuously powered wake-up radios have a fast response to a request for a communication session but at the expense of high power consumption or possibly poorer performance in lower power consumption configurations. The preferred option in low power applications is to use the periodic sniffing technique in which the node is in a very low power sleep mode with a timer that periodically starts the wake-up radio, which then sniffs for a wake-up transmission from the hub on each of the potential channels. If a valid wake-up signal is detected then the wake-up radio may instruct the remainder of the node to startup and engage in a full communication session. The time between sniffing is typically set to satisfy the application's minimum latency requirements.
A wake-up operating system comprises two types of entity: a wake-up instigator and a wake-up receiver. The wake-up instigator is the device that transmits the wake-up request whilst the wake-up receiver is the device receiving the wake-up request. There may exist more than one wake-up receiver for a given wake-up request. A wake-up receiver is typically part of a battery powered device or secondary node with low power operating constraints that may perform sensing or control functions as part of the application of the wireless system. A wake-up instigator is typically a hub (or primary node) that connects to remote secondary nodes within a wireless network and often has connectivity to the Internet to facilitate the flow of information and control of the wireless network to users.